1. Field of the Invention
The present invention relates to a measuring method for measuring a rotational angle of a servo motor that drives a spot welding gun, and a measuring device that performs such a method.
2. Description of the Related Art
Generally, a spot welding gun includes a movable electrode that is driven by a servo motor, and an opposite electrode disposed in opposing relationship to the movable electrode. Position control of the movable electrode is performed based on a rotational angle of the servo motor when a distal end of the movable electrode and a distal end of the opposite electrode contact or abut each other.
However, during a spot welding operation, sometimes the distal end of the movable electrode and/or the distal end of the opposite electrode is deformed or worn out each time welding is performed or an oxide film may adhere to the movable electrode and/or the opposite electrode. Therefore, it is not possible to flow a welding current efficiently through the movable electrode and the opposite electrode.
Thus, it is necessary to periodically dress the surfaces of the movable electrode and the opposite electrode, but the distal end position of the movable electrode and the distal end position of the opposite electrode are changed due to the dressing operation. Since a position control of the movable electrode is performed based on contact rotational angle as described hereinbefore, it is necessary to detect again the contact rotational angle after the dressing operation.
In Japanese Laid-open Patent Publication No. 2002-283059, a step of pressing a movable electrode against a fixed electrode until a load imparted to a distal end of the movable electrode reaches a first threshold value, and a storing step of storing, as a reference position, a position of a shaft of the movable electrode when the movable electrode is retracted after the load reached the first threshold value and the load imparted to the distal end of the movable electrode reaches a second threshold value smaller than the first threshold value is disclosed.
Further, in Japanese Patent No. 4,880,021, a minimum torque limit is obtained which enables a movable electrode to operate. When the torque limit is reached as a result of the movable electrode contacting an opposite electrode, the movable electrode is stopped.